Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB

Results from the first experiment at the Facility for Rare Isotope Beams (FRIB) will be presented, revealing a new microsecond isomer in 32Na, two nucleons outside 32Mg at the heart of the island of inversion [1, 2]. A cascade of two delayed gamma rays at 224 keV and 401 keV were observed following implanted 32Na ions, indicating the presence of a 24(2)-μs isomeric state. This nucleus is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories.

The 625-keV isomer is interpreted as either a 6 − spherical shape isomer decaying via a hindered E2 transition, or a 0 + spin isomer decaying by M2. State-of-the-art calculations favour the latter explanation, indicating that the low-lying states of 32Na are dominated by deformation.

[1] H. L. Crawford, V. Tripathi, J. M. Allmond et al., Phys. Rev. Lett. 129, 212501 (2022)

[2] T. J. Gray, J. M. Allmond, Z. Xu et al., Phys. Rev. Lett. 130, 242501 (2023)